Reservoir stabilization by treating water sensitive clays

ABSTRACT

A method of treating subterranean formations containing water-sensitive montmorillonite clays by contacting the formation with an aqueous solution of potassium salts of organic acids at elevated temperatures whereby the montmorillonite is transformed into other clay minerals which are less sensitive to swelling when contacted with water.

FIELD OF THE INVENTION

This invention relates to an improved method for the treatment ofclay-containing formations. More particularly, this invention relates toa method of treating subterranean formations containing water-sensitivemontmorillonite clays by hydrothermal means to increase waterpermeability and to stabilize the clays contained therein.

DESCRIPTION OF THE PRIOR ART

In modern day production of hydrocarbons from subterranean formations,it has become fairly common practice to apply secondary recoverytechniques to the hydrocarbon bearing formations to recover additionalquantities of hydrocarbons therefrom. Among the methods employed insecondary recovery operations are those involving water, steam or gasflooding or combinations thereof. One of the more serious problemsencountered in secondary recovery operations with water or hydrothermalprocedures, such as steam flooding, is that an undergroundhydrocarbon-bearing formation also may contain clay or claylike bodieswhich swell or disperse on contact with water. In particular, thepresence of hydratable clays, such as montmorillonite, often results insubstantial decreases in permeability when water comes into contact withthe formation. Thus, the secondary recovery processes involvingtreatment of such a formation with water or low quality steam generallyresult in swelling of the clays by adsorption of the water with theconcomitant result that the permeability of the formation is materiallydecreased. The decrease in the permeability of the formation to waterresults in the reduction of the amount of hydrocarbon which may berecovered therefrom by means of the secondary recovery procedure.

Many techniques for stabilizing water-sensitive formations have beendevised which involve minimizing total interparticle swelling by theintroduction of exchange cations such as calcium. Other techniquesinvolve the use of organic cations, such as quanidine hydrochloride,which are adsorbed on the clay thereby preventing hydration andswelling. However, the use of both organic and inorganic materials,which depend for success upon the exchange capacity of the clays arelimited by the fact that the mineralogical structure of thewater-sensitive clays is not altered and hence the stabilization is nottruly irreversible, especially at higher temperatures.

More recently, the effects on the mineralogy of formation matrices ofhydrothermal treatments, such as steam flooding, have been reported. Forexample, in formations containing quartz, nonexpandable clays, such askaolinite, and traces of carbonate minerals, hydrothermal treatments maydissolve a portion of these minerals and convert a further portion intoa new expandable form of clay mineral known as montmorillonite. Thisnew, synthesized montmorillonite, owing to its water adsorption andswelling properties, is expandable and thereby causes a decrease inpermeability and porosity of the formation, with a resultant decrease inhydrocarbon production from the formation. Since the majority ofhydrocarbon-bearing formations contain minerals which are capable ofsuch montmorillonite synthesis, the inhibition of these undesirablereactions is important when considering hydrothermal processes insecondary recovery.

The U.S. Pat. No. 3,384,177, to Day et al, discloses a method wherebythe transformation of kaolinitic clays into water-sensitivemontmorillonite can be inhibited through the use of ammonium hydroxidein clay containing sandstones. This patent teaches that themontmorillonite can be converted into a nonexpandable clay whensubjected to an ammonia treatment at elevated temperatures, whichthereby minimizes adverse swelling during subsequent contact of theformation with aqueous media.

U.S. Pat. No. 3,543,858 discloses a method for transformingmontmorillonite to nonexpanding illitic and lesser expanding mixed layerclays by treating the formation with a heated aqueous solution ofpotassium chloride. In this way, the montmorillonite can be renderedinsensitive to aqueous media by transforming, irreversibly itsmineralogical structure.

SUMMARY OF THE INVENTION

This invention provides a method of mineralogically transforming aformation matrix by injecting thereinto an aqueous solution of apotassium salt of an organic acid at elevated temperatures, continuingthe injection until the formation is heated to an elevated temperaturefor a desired length of time when stabilization of the formation iseffected to a desired radial distance from the well bore.

DESCRIPTION OF THE INVENTION

With the method of this invention, clay-containing formations areeffectively transformed to stabilize their swelling upon subsequentexposure to aqueous media, such as water, hot water, or steam. Themethod has utility as a pretreatment operation for any operation, suchas well stimulation or secondary recovery, wherein a hydrothermaltreatment is to be undertaken. For example, a subsequent steam floodingoperation, by means well known in the art, may be accomplished withoutthe normally associated clay swelling and/or dispersion. Suchpretreatment permits more effective recovery of the hydrocarbonscontained in the formation, since blocking and plugging of the formationby the flooding fluid is precluded or retarded, thereby allowing theeasier penetration of the formation by the flooding fluids.

The invention herein disclosed provides an improved method for thestabilization of subterranean formations. Exposure of the formation totemperatures of from about 260° C. to about 310° C. and an aqueoussolution of a potassium salt of an organic acid having a concentrationof about 0.5 N to about 3.0 N produces a transformation of thewater-sensitive montmorillonite clays to a more stable illitic-typeclay.

The following examples based on the results of tests are illustrative ofthe method of this invention. Mixtures consisting of 1.5 grams ofdolomite, 1.5 grams of kaolinite and 1.5 grams of quartz sand wereground to pass a 200 mesh screen. Portions of these mixtures were thenplaced in stainless steel bombs and thereafter a treating solution wasadded. The treating solutions used were 100 ml of 0.5 N potassiumacetate, or 0.5 N potassium formate or 0.5 N potassium biphthalate indistilled water.

The bombs were sealed and then maintained at a given temperature for agiven period of time, after which the solid products were analyzed formineral composition by x-ray diffraction. The results from these testsare given in Table I wherein the mineralogical analysis, in percent byweight, is given for the samples before and after treatment.

The results demonstrate that treatment with the potassium formate,potassium biphthalate and potassium acetate at 300° C. for seven dayscaused transformation of the swelling clay (montmorillonite) tononswelling clay (illite) and lesser swelling clays (mixed-layer).

It has been demonstrated that an aqueous solution of a potassium salt ofan organic acid transforms the montmorillonite to illitic andmixed-layer clays upon hydrothermal treatment. Thus, there has beenshown an improved method of treating a subterranenan hydrocarbon-bearingformation by hydrothermal means in order to stabilize thewater-sensitive clays therein.

Other modifications and variations, as hereinbefore set forth, may bemade without departing from the spirit and scope thereof, and thereforeonly such limitations should be imposed as are indicated in the appendedclaims.

                                      TABLE I                                     __________________________________________________________________________                  CONDITIONS                                                                    TIME                                                                              TEMP    MINERALOGICAL ANALYSIS, PERCENT BY WEIGHT           TREATING SOLUTION                                                                           (days)                                                                            (°C.)                                                                          Q   M    I   C   D    F   M-L                       __________________________________________________________________________    0.5 N                                                                             potassium acetate                                                                       7   300 Before                                                                            50  50                                                  in distilled water                                                                              After                                                                             65  --   28  --  --   7   --                        0.5 N                                                                             potassium formate                                                                       7   300 Before                                                                            50  50                                                  in distilled water                                                                              After                                                                             52   5   18  --  --   20  5                         0.5 N                                                                             potassium biphtha-                                                                      7   300 Before                                                                            50  50                                                  late                                                                          in distilled water                                                                              After                                                                             42   5    7  26  20   --  --                        __________________________________________________________________________     Notes:                                                                        Q = Quartz;                                                                   M = Montmorillonite;                                                          I = Illite;                                                                   C = Calcite,                                                                  D = Dolomite;                                                                 F = Feldspar                                                                  M-L = Mixed-Layer (illite-montmorillonite).                              

I claim:
 1. A method of altering the matrix of a hydrocarbon-bearingformation having present therein montmorillonite clays, comprising thesteps of introducing via a wellbore penetrating said formation a heatedaqueous solution of a potassium salt of an organic acid, continuinginjection of said solution until said matrix is heated to a minimumtemperature of about 260° C. for a desired radial distance from saidwellbore, whereby said matrix is stabilized by mineralogically alteringsaid montmorillonite clays.
 2. A method as claimed in claim 1 whereinsaid potassium salt of an organic acid is potassium acetate.
 3. A methodas claimed in claim 1 wherein said potassium salt of an organic acid ispotassium formate.
 4. A method as claimed in claim 1 wherein saidpotassium salt of an organic acid is potassium biphthalate.
 5. A methodas claimed in claim 1 wherein said clays in said formation aretransformed to non-hydratable clays.
 6. A method as claimed in claim 1wherein the concentration of said potassium salt of an organic acid isin the range of from about 0.5 N to about 3.0 N.
 7. A method as claimedin claim 1 wherein said aqueous mixture is injected at a temperature inthe range of from about 260° C. to about 310° C.